The application of electron-bombardment ion thruster systems has been analyzed in detail for a broad set of planetary and near earth missions. Reference is made to Atkins, K. L., "Mission Applications of Electric Propulsion", AIAA Paper 74-1085, October 1974; Duxbury, J. H. and Finke, R. C., "A Candidate Mission Using the Shuttle and Solar Electric Propulsion", AAS Paper 75-163, 1975; Guttman, C. H., et al, "The Solar Electric Propulsion Stage Concept for High Energy Missions" AIAA Paper 72-465, April 1972; and Sauer, C. G., Jr., "Trajectory Analysis and Performance for SEP Comet Encke Mission", AIAA Paper 73-1059, October, 1973, for studies dealing with planetary missions and to "Payload Utilization of SEPS", Report D-180-19783-1, Boeing Aerospace Co., 1976; "Concept Definition and Systems Analysis Study for a Solar Electric Propulsion Stage", Report SD74-SA-0176-1, Rockwell International, 1975; and Stearns, J. W., "Large-Payload Earth-Orbit Transportation with Electric Propulsion", (JPL-TM-33- 793, Jet Propulsion Lab.; NAS 7-100), NASA CR-148973, 1976 for studies dealing with near earth missions. A further thruster system of interest is the 30-cm diameter mercury bombardment ion thruster presently under development by the Lewis Research Center and discussed in Schnelker, D. E. and Collett, C. R.: "30-cm Engineering Model Thruster Design and Qualification Tests", AIAA Paper 75-341, March, 1975.
As discussed in the mission studies referred to above, ion thrusters are typically powered by solar arrays used in conjunction with power processing equipment. The function of the power processing equipment is to match the thruster load requirements to the solar array. For the high power, high voltage levels encountered with thruster loads, such power processors are heavy, complex, expensive and are a substantial burden to the spacecraft thermal control system. Considerable efforts have been and are being made to develop alternate power processing concepts to reduce these spacecraft penalties.
One of the alternate concepts under consideration is the Integrally Regulated Solar Array (IRSA). The IRSA system provides regulated DC power from a controlled solar array directly to its loads without an intervening power processor. Reference is made to Triner, James E., "A Digital Regulated Solar Array Power Module", NASA TM X-2314, 1971; and Gooder, Suzanne T., "Series-Parallel Method of Direct Solar Array Regulation", NASA TM X-73505, 1976 for a discussion of such a system. An integrally regulated solar array has been used to power a 30-cm ion thruster beam load as described in Gooder, Suzanne T., "Evaluation of 30 cm Ion Thruster Operational Campatibility with Integrally Regulated Solar Array Power Source", NASA TN D-8428, 1977. It has been determined, based on this latter study, that the basic characteristics of solar arrays (such as ripple and the inherent current limited output) are well matched to thruster requirements. To explain, integrally regulated solar arrays typically comprise blocks of cells that are automatically switched in or out by closed loop control so as to provide voltage or current regulation. When this method of regulation is used, the cells that are switched out become dead weight, resulting in a higher subsystem mass to power ratio. Minimization of the solar electric propulsion system mass to power ratio is clearly of primary importance and hence it is desirable that all available solar array power be used. It will, therefore, be appreciated that it is highly desirable to operate the solar array at the maximum power point under all conditions of operation.
Systems capable of operating a solar array at its maximum power point are discussed in Gruber, Robert, "High Efficiency Solar Cell Array Peak Tracker and Battery Charger", IEEE Power Conditioning Specialists Conference, Inst. Electr. Electron Eng., Inc., 1970, pp. 128-138; Paulkovick, John and Rodriguez, G. E., "Maximum Power Transfer by Conductance Comparison", IEEE Power Conditioning Specialists Conference, Inst. Electr. Electron Eng., Inc. 1970, pp. 114-127; and "Advanced Voltage Regulator Techniques as Applied to Maximum Power Point Tracking for the NIMBUS Meteorological Satellite" (AED-R-3221, Radio Corp. of Am.; NAS5-3248), NASA CR-b 93413, 1967. However, these systems suffer the serious disadvantage that batteries are incorporated for energy storage. Optimum solar electric propulsion systems do not use batteries to augment beam power because of the added weight. For most systems without batteries, considerations concerning system dynamics make the implementation of an automatic maximum power point tracker a difficult problem.
Techniques for maintaining a solar array at its maximum power point fall into two categories: (1) open loop and (2) closed loop. Open loop systems measure one or more array parameters and then predict or determine the array maximum power point. Moreover, a system using this technique could include a small reference array. Closed loop techniques determine the location of the maximum power point directly and maintain the system at that point. With open loop techniques it is generally not practicable to accummodate unpredictable changes in solar array output such as those caused by severely degraded cells. Exemplary closed loop systems are disclosed in Gardner, J. A., "Solar Electric Propulsion System Integration Technology (SEPSIT)" Volume 1, Technical Summary, (JPL-TM-33-583-Vol-1, Jet Propulsion Lab.; NAS7-100), NASA CR-13071, 1972; Gardner, J. A., "Solar Electric Propulsion System Integration Technology (SEPSIT)", Volume 2, Encke Rendezvous Missions and Space Vehicle Functional Description, (JPL-TM-33-593-Vol-2, Jep Propulsion Lab., NAS7-100), NASA CR-130702, 1972; and Gardner, J. A., "Solar Electric Propulsion System Integration Technology (SEPSIT)", Volume 3, Supporting Analyses, (JPL-TM-33-583-Vol-3, Jet Propulsion Lab.; NAS7-100), NASA CR-130703, 1972. The closed loop techniques for ion thruster systems disclosed in the referenced studies have been limited in performance because of constraints regarding system dynamics and power dissipation.
Other references of possible interest include the following U.S. Pat. Nos. 3,416,319 (Rubenstein); 3,562,581 (Sonjra); 3,566,143 (Cherdak et al); 3,577,734 (King); 3,626,198 (Boehringer); 3,909,664 (Waskiewicz etal); 3,999,100 (Dendy). The Boehringer patent disclosed an arrangement for maximizing the power output of a solar cell generator, such as found in a satellite, wherein the current and voltage are multiplied together to form a power figure, the current and voltage being varied to locate a desired point on the power curve. The Waskiewicz et al. patent discloses a plasma spraying apparatus where current and voltage are combined to form a power signal which is compared with a power reference to produce a control signal which is used in control of the apparatus. The remaining patents disclose various circuits and systems of interest including solar powered ion thrusters, further control circuits for ion devices and power control circuits for other devices.